Hitherto, road information such as lane information and curve information on the road was represented by lane marks for distinguishing the lanes, road signs and others, and was visually recognized by the vehicle drivers.
In visual recognition of lane marks and signs, however, it was hard to obtain the road information accurately for the vehicle drivers when driving in bad weather, driving at night, or driving in a tunnel, and hence the safety was impeded. Accordingly, it has been attempted to run by giving road information by means of various markers and the like.
As one of such examples, recently, by burying a magnetic material in the road as a road marker, the marker is detected by a detector mounted aboard a vehicle, and the position of the vehicle on the road is detected, and it is attempted to allow the vehicle to run.
In this case, the magnetic flux density in the horizontal and vertical direction from the magnetic material marker buried in the road was detected by mounting a magnetic sensor on the vehicle. Usually, the marker of magnetic material is a magnet. Hence, the detector by the magnetic sensor is hard to keep balance between the directivity and detecting distance. When the detecting distance is long, a magnet of strong magnetic force and large size is needed and it is not economical. Besides, a magnet having a strong attracting force may attract iron particles or cans scattered about on the road.
In a related art, ferrite and ferromagnetic amorphous materials are known to induce dimensional changes called Joule effect due to application of external magnetic field (called magnetostrictive phenomenon). At retail stores, by adhering the magnetostrictive resonator having such property to the merchandise, the magnetostrictive resonator detection apparatus is installed at the entrance and exit of the store, and illegal take-out of merchandise is prevented.
When an alternating-current magnetic field or electric field is applied to the magnetostrictive resonator by electromagnetic wave, an electromagnetic wave having a specific phase difference from the applied (call) electromagnetic wave is radiated from the magnetostrictive resonator. So far, the presence of the magnetostrictive resonator was sensed by detecting the phase difference between the call electromagnetic wave and the electromagnetic wave radiated from the magnetostrictive resonator.
In the conventional method of detecting phase difference, as compared with the transmission output level of the called electromagnetic wave, the input level of electromagnetic wave radiated from the magnetostrictive resonator is very small. It was hence difficult to detect the input phase by reference to the output phase. Usually, the ratio of signal level of transmission output to input is about one-millionth. Or, the reception section may be saturated by transmission output, and it was hard to detect a feeble input signal, as compared with transmission output. In this detecting method, therefore, in order to apply to a road traffic system, it was not easy to obtain the detecting distance and directivity, and there were problems in the aspect of practical use.